Semiconductor power switches, for example MOSFET devices, are frequently used to control the flow of current within an electronic device, and in particular to control the supply of current through an inductive load, such as those used in large motors and generators.
By way of illustration a MOSFET device 100 configured to operate as a switch for controlling a current through an inductive load 101 is shown in FIG. 1, where the drain 102 of a MOSFET device 100 is coupled to a voltage supply Vcc via the inductive load 101 and a resistive load 104, the source 105 of the MOSFET device 100 is coupled to ground and the gate 106 of the MOSFET device 100 is coupled to a control signal for switching the MOSFET device on or off (i.e. cause the drain/source to become closed or open circuit).
For an N-channel MOSFET device a positive control voltage will cause the MOSFET device to turn on, for a P-channel MOSFET device a negative control voltage will cause the MOSFET device to turn on.
As is well known to a person skilled in the art, the source and drain of a MOSFET device are formed in a semiconductor material such as silicon, while the gate is formed from a conductive material, such as polycrystalline silicon. The gate is separated from the semiconductor material by an insulating layer, for example silicon dioxide. As such, a MOSFET device is susceptible to damage when a breakdown voltage is applied to the MOSFET.
Two types of voltage damage that can occur to a MOSFET device are electro static discharge ESD and electrical over stress EOS.
In the case of EOS there are three possible failure modes. First, a breakdown voltage of the gate oxide is reached; second, a breakdown voltage of the drain to source BVDSS junction is reached; and third, a maximum junction temperature is reached due to high temperature generated by energy discharges.
In the case of ESD there are two possible failure modes. First, the breakdown voltage of the parasitic bipolar transistor is reached; and second, a breakdown voltage of the gate oxide is reached.
One solution that has been adopted to avoid a voltage that could damage the semiconductor power switch involves the use of a zener clamp 200, where the anode of the zener clamp 200 is coupled to the gate 106 of the MOSFET device 100 and the cathode is coupled to the drain 202, as shown in FIG. 2.
The zener clamp 200 (i.e. zener diode) is chosen to have a breakdown voltage below that of the maximum drain to source voltage. As such, if the zener clamp breakdown voltage is applied to the cathode of the zener clamp current is caused to flow through the zener clamp from the drain to the gate, resulting in the MOSFET device switching on and allowing current to flow from the drain to the source, thereby allowing the voltage at the drain to be reduced and consequently avoid damage to the MOSFET device.
As such, this solution provides a means for clamping the voltage generated at the drain of the MOSFET device to a predetermined voltage (i.e. the breakdown voltage of a zener diode).
However, typically, the breakdown voltage of a zener clamp is relatively low compared to the maximum drain to source voltage of a MOSFET device. As such, to allow an appropriate clamp voltage to be selected a zener clamp comprising a plurality of zener diodes placed in series is needed. Consequently, this solution can result in a voltage clamp circuit being relative large in size. Further, the coupling of zener diodes in series can make it difficult to provide an accurate clamp voltage.
Additionally, as the switching characteristics of the zener diode are slow they are not suitable for providing ESD protection to a MOSFET device. As such, additional ESD protection circuitry is required, thereby resulting in a further increase in size and complexity of a switching circuit.
U.S. Pat. No. 5,812,006 discloses an optimized output clamping structure that includes a power output transistor having a first breakdown voltage and a breakdown structure having a second breakdown voltage coupled to the power output transistor. The second breakdown voltage is less than the first breakdown voltage and follows the first breakdown voltage across all temperature and semiconductor process variations. Notably, the source and drain doping profiles of the power MOS are used to create a switch device (NPN or MOS) in order to protect ‘circuits’. Thus, US005812006A discloses a diode that used to clamp a MOSFET, which operates as a diode during electrostatic discharge, thereby failing to protect against ESD. Furthermore, there is no solution to integrate a clamp inside the MOSFET.
It is desirable to provide a semiconductor switch arrangement and an electronic device that provides improved protection against electrostatic discharge.
Statement of Invention
The present invention provides a semiconductor switch arrangement and electronic device as described in the accompanying claims.
This provides the advantage of providing a single voltage clamp device that can provide protection to a semiconductor power switch, for example a MOSFET, insulated gate bipolar transistor IGBT, gate turn off thyristor GTO, or power bipolar transistor, from both electrostatic discharge and electrical over stress (EOS) like energy discharges.
Further, it allows a reduction in die size and improved voltage clamp accuracy.